Coating is typically used to improve an article's durability, corrosion resistance, visibility and appearance. Examples of materials that require coating include thermoplastic polyolefins (TPOs), metals and composites, including but not limited to, those metals and composite materials used in the auto, marine and aviation industries. Some of the materials that require coating present challenges to coating, such as being difficult to coat, requiring multiple coats or requiring pre-treatment of the material.
TPO's such as polyethylene, polypropylene (PP), polystyrene (PS), thermoplastic elastomer (TPE) or ethylene-propylene rubbers have steadily grown in use as a material of construction for a large array of consumer goods. In particular, polyethylene (PE) is used in the manufacture of toys, containers, packing films, household appliances, and in particular, the automotive industry as it exhibits the primary properties of being relatively inert, flexible, impermeable to water vapour, light weight, durable and is produced at a relatively low cost.
However as PE is non-polar, it is reluctant to accept a coating of paint or decorative print coating. Most paints are polar, and thus require a surface with some degree of surface polarity before they can adhere to it with any degree of desirable adherence. Conventional approaches to this problem have had limited success.
One approach to enhancing the paintability of a TPO surface or substrate is to subject it to physical or chemical etching. For example, the outside surface of PE water tanks are either lightly sanded or flame treated to remove the waxy, glossy surface prior to painting. Alternatively, the TPO surface or substrate may be irradiated with a plasma. A variation to this method is taught in JP59114039A2, whereby a primer is applied to a non-vulcanised rubber substrate and dried prior to irradiation. While generally effective, these methods are complex in nature and are more difficult to control in terms of quality and consistency across the surface. In addition, these techniques are generally more expensive than other alternatives.
Another approach is to apply several primer coats to a TPO substrate. While such primers are generally recognized as effective over short durations, they are expensive and their application is an extra step in the finishing of the manufactured TPO article. In addition, primer systems require multiple coats and still lack sufficient bonding strength to maintain adherence to the TPO substrate over time. Known primer compositions are unable to achieve satisfactory adhesion, resulting in peeling or chipping of the primer and/or covering paint over time.
As conventional primers have had limited success, more recent techniques have been to modify the physical and/or chemical properties of the TPO per se (e.g. as taught in WO 9325617). This can be achieved either by blending the thermoplastic polyolefin with other thermoplastic polymers, or by grafting functionalised groups to one or more polar groups along the polymeric backbone. While this approach has been successful, it is complex and can only be applied to existing substrates at the time of manufacture.
Metals and composite materials, including but not limited to metals and composites used in the auto, marine and aviation industries, also require coating.